Carsten, are you running the example code from here?
Arduino Playground - Servotimetimer1If you have made any changes to that sketch or are runing something else, could you post the code you are running.
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Its not about the pulse-length, the original code works (so does my little robot now, following a light) but as soon I added some delay() it fails.
I cant check right now, but as soon as I deleted the delay() lines it started to work.
Carsten